VCOs are an important part of many electronics circuits and are particularly important in communication circuits. For example, VCOs may be used to generate local oscillator signals used by transceiver subsystems for frequency up-conversion and down-conversion. VCOs are also used to generate clock signals. Multiple VCOs may be present in a wireless device for such functions.
A VCO is typically designed to operate at a specific frequency or in a specific range of frequencies. The frequency of oscillation for the VCO may deviate from a nominal design value due to various factors such as circuit component tolerances, integrated circuit manufacturing process variations, circuit component aging, etc. The oscillation frequency may also drift during normal operation due to various factors such as temperature, power supply variation, etc.
The VCO may include a programmable capacitor bank to aid with the adjustment of the oscillation frequency. The capacitor bank may contain a bank of tuning capacitors that may be individually switched on or off. Each tuning capacitor may adjust the oscillation frequency when switched on. The tuning capacitors typically have binary weighted capacitances. Different total tuning capacitances may be obtained by switching on the appropriate tuning capacitor(s).
One method for compensating for frequency drift is to periodically relock the VCO using the capacitor bank. However, the periodic relock may occur too frequently, resulting in loss of air time because, for example, each relock may take as long as 100 μs to complete. Alternatively, the periodic relock may be too infrequent, resulting in unacceptable performance degradation due to frequency drift.